1. Field of the Invention
This invention relates to inverse synthetic array radar (ISAR); more particularly, it relates to an ISAR system and method for improving the resolution of an ISAR image by providing compensation for non-uniformity in the magnitude of the angular velocity of a rotating target as the target rotates to generate the synthetic-aperture-angle.
A target ship moving in pitch can take many seconds, e.g., more than five seconds, in the course of rotating between the point at which its bow is deepest in the water to the opposite point at which the bow is up. At each such opposite end of a half pitch cycle, the magnitude of the pitch component of the ship's angular velocity is zero. At some point near the midpoint, the magnitude has a maximum value.
As a result of such non-uniformity in the magnitude of the angular velocity, a given scatterer causes a non-uniform or time-varying doppler shift in the radar pulses it reflects.
2. Description of the Prior Art
Prior art ISAR systems have been organized to provide short, overlapping processing intervals in an effort to deal with the problems arising from non-uniformity in the magnitude of the angular velocity of a rotating target.
Such a "short" processing interval constitutes an interval short enough to be consistent with the angular motion of the target remaining constant, (i.e., within narrow limits) throughout the processing interval. If the angular motion remains constant throughout the processing interval, a given scatterer will produce a doppler shift that remains constant throughout the processing interval. In other words, a scatterer will not drift from one doppler filter to the next if the processing interval is short enough. Because of the use of such short processing intervals, images produced by such prior art ISAR systems have less smear than would otherwise result from scatterer drift. However, this advantage of reduced smear has been gained as a compromise that entails a countervailing disadvantage of loss of resolution.
A substantial need has existed for an improved ISAR system and method to provide better resolution so that more detailed and more easily interpreted ISAR images can be displayed, and to provide such better resolution without sacrificing performance in another important respect such as performances in preventing excessive smear.
This substantial need has been a longstanding one, and has received extensive attention for numerous reasons including the widely recognized desirability of improving resolution.